Last updated: May 25, 2018 | See all Documentation
Let’s Encrypt uses the ACME protocol to verify that you control a given domain name and to issue you a certificate. To get a Let’s Encrypt certificate, you’ll need to choose a piece of ACME client...

https://zerossl.com/ seems to be a popular one. Just click “Online tools” and “Start”, fill in the form and follow the instructions. Then consult the documentation @JuergenAuer linked above for installing the resulting certificate on your server. Remember, Let’s Encrypt certificates are valid for 90 days, so you’ll need to repeat this manual process to renew your certificate at least that frequently (best to renew early to leave some time to recover in case of problems).